There is provided a system in which a transponder is carried by each vehicle traveling on or above the earth's surface which transmits a signal which is responsive to an interrogation signal to a ground station through two or more satellites and in the ground station, the position of the vehicle is determined from the propagation time differences of the response signals received via the satellites. In this system, the information indicative of the time lag which is required for the transponder to receive the interrogation signal and to transmit the response signal responsive to the interrogation signal is included in the response signal and then such response signal is transmitted. Thus, the time lag which is peculiar to each transponder and corresponds to the time required from the reception of the interrogation signal to the transmission of the response signal can be corrected. With this system, the positioning accuracy of each vehicle can be improved.